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I got two quotes for replacing my 4 ton outside unit that has a 40 Amp service wih a 5 ton unit. One quote for the Trane XL19i said to connect to existing electrical. Second was for a Carrier Infinity 18 model 38TDB but stated that a 50 amp service is needed. Who's right?

Have each contractor show you the manufacturers specification sheet on the particular unit (model and size) that they are installing and it will indicate the minimum circuit amps that are required for the unit. There should be no doubt. Make sure if a larger breaker is installed in your panel, that it doesn't create an overload for your panel. Also be aware that in some areas, A LICENSED ELECTRICAL CONTRACTOR is required to change that breaker. Sorry, but that is the code (law) in some areas and it's that way for your protection. Some HVAC contractors may also have a licensed electrician on staff, but make sure whoever does the work acquires all the necessry permitting and the work is inspected upon completion. If in doubt, make a clarification that you will hold 10% until local code enforcement performs their inspection and gives it their seal of approval.

Next question. Why are they upsizing the system, and can your duct system accomodate the added airflow required with the larger system? Remember, bigger is not necessarily better.

Each contractor, particularly in changing the size of the system should have perform a load calculation (manual J) to determine exactly what size system SHOULD be in your home.

Trane guy did perform load calculation and measured all rooms and size of ducts. His calculation said 4.6 tons. Current sytem during hot weather would kick on at 11am and run continious until 11pm at night. System could not keep up and indoor temp would creep up to 82 degrees. Current system checked on two ocasions by defferent companies for correct pressures and if condenser and filter was clean. Both said that unit was too small. Having two large shade trees on West side of house die on me did not help.

I have a problem with a unit that would run from 11AM to to 11PM and let the temp creep up to 82 degrees, and then hear that that unit is a four ton and the load calc said it needed a 4.6 ton, I'm not going to sit here and do the math but I can tell you that that does not make any sense common or otherwise. If the load calc was correct then there is something wrong with the existing unit and that should be addressed first before you spend a lot of money and end up with the same problem again, ask the Trane guy to clear that up for you.

My point is that you are going to switch from a 4 ton to a 5 ton and the problem is probably in your ductwork.

Ten bucks says that fats is right. Many believe Manual J oversizes. A study by Proctor Engineering Group showed that J7 oversized by 30% and that contractors added another 20% of unnecessary fudge factors. J8 is rumored to oversize by more. Of course those systems did not output 50% more due to poor installation practices, etc. My crystal ball says that airflow is pathetic and the ducts are leaky. There's probably some minor to major building envelope issues as well.

On the other hand, my crystal ball has been wrong before. You could be right Mr. Smith. But if fats is right, then it could be that if there are leaky ducts and poor airflow then adding a ton of capacity might net a delivered increase in capacity of half that amount.

Fat eddy, I understand what you are saying, there's only so much air that can be forced through a certain size duct. The entire system is being changed out, condenser, A coil and furnace so it will be a matched set. Same duct work though. Condenser is 23 years old and furnace is 18. Entire first floor is vaulted ceiling with no attic and little ventilation up there. Ceiling gets pretty warm in summer. Downstairs is walk out basement and 1/3 size of upstairs. Going to install two zones so this may help also.

one needs to know the full load amps, the locked rotor amps, the length of run, how | where any circuit is to be installed. The use of other than 60C rating of wire is wasteful each & every time the unit runs.

Originally posted by cem-bsee one needs to know the full load amps, the locked rotor amps, the length of run, how | where any circuit is to be installed. The use of other than 60C rating of wire is wasteful each & every time the unit runs.

cem-bsee

first sentence made alot of sense. second sentence got a little off track. if you only used 60C wire, you would have to upsize wire, some of the time, after you derated it for the hot attic space. 75c & 90c wire might allow you to use the normal wire size after derating. not always, but those attics can be really hot in the summer. in other words, for example, I would rather use #6 75c, than use #4 60c. would be cheaper and easier. copper cost more than insulation.

Isn't it ironic how most people think that changing all the equipment without consideration to the ductwork will be all that is needed. Manufacturers make very efficient equipment. Contractor and homeowner agree on complete equipment changeout. The homeowner now has the best efficiency in equipment but a very poor delivery system because no one considered the duct work. Its like delivering pure clean water in a dirty leaking bucket. Oh and another irony, All new equipment and still the same ol $1.25 air filter, I call it an air strainer.